Dairy production absolutely requires raising animals for slaughter. There is no beef industry without the dairy industry. Like mammals, cows produce milk as food source for their offspring - the dairy industry ensures that cows are always producing offspring. Those offspring don't get milk - it's sold to us. Those male offspring become veal and are slaughtered early. Those female offspring become more dairy cows. Those dairy cows that can no longer produce milk are slaughtered for meat and myriad other uses.

That's a huge reason why veganism is growing far faster than vegetarianism. People are realizing the systemic connection between the industries and understanding that all animal products must be eschewed.